The present invention relates to a multipath detector for an FM tuner.
The multipath is the propagation phenomenon that results in signals reaching an antenna by two or more paths including at least one reflection from some object, causing distortion in radio signals.
When the multipath which is also called multipath transmission generates, the amplitude of the FM signal produced from an intermediate frequency amplifier is modified to generate an alternating current component in the FM signal. The multipath detector detects the alternating current component.
FIG. 3 shows a conventional multipath detector in the FM tuner. An intermediate frequency (IF) obtained through a front end (not shown) is amplified through an intermediate frequency amplifier 1 and applied to an FM detector 2 where a composite signal is obtained.
The composite signal is demodulated by a stereo demodulator (not shown) to extract a main signal (L+R) and a sub signal (L-R) to provide right and left signals.
Another signal from the amplifier 1 is applied to a level detector 3 which detects a level of the signal. An output signal Vs from the level detector 3 is applied to a signal meter to detect the strength of the received signal. The signal meter signal Vs is also applied to a high-pass filter 4 where the alternating current component included in the signal meter signal Vs is extracted as a multipath signal. The multipath signal is applied to a level detector 6 through an amplifier 5. The level detector 6 detects a level of a multipath to produce a multipath level signal which is displayed, for example by a multipath meter. The level detector 6 has a detecting level V2 in order to detect the multipath. The detecting level V2 is lowered from a level of electric field V1 by a value X as shown in FIG. 4.
Hereinafter a preferable relationship between V1 and V2 will be described. The signal meter signal Vs includes also another alternating current component generated by the frequency modulation and selecting elements such as ceramic filter and coil. Such an inherent component must not be filtered. Therefore, the detecting level V2 must be larger than the inherent component.
Here, if the threshold is V(x), EQU V(x)=V1-V2 (1)
Since the signal meter has a logarithmic characteristic, EQU V1=A.multidot.log.sub.10 (VIN+VO) (2)
where A is a constant, VIN is the input level of electric field, and VO is the residual noise level.
Similarly, EQU V2=A.multidot.log.sub.10 (VIN(1-x)+VO) (3)
Here, the value x represents a degree of amplitude modulation of the level V1.
Therefore EQU 0.ltoreq.x.ltoreq.1 (4)
From the equations (1), (2) and (3), ##EQU1##
Since A'1 =A/log.sub.10 e, VIN 0, (1-x).gtoreq.0, and x.gtoreq.0, if the value x is x.noteq.0, EQU dV(x)/dVIN&gt;0
Namely, if the level VIN is large, the level V(x) corresponding to the amplitude modulation degree is large. Consequently, it will be understood that it is necessary to increase the threshold with an increase of the electric field level. Otherwise, it is impossible to properly detect the multipath.